This invention relates in general to selective call radios, and particularly, to a selective call radio having an integrated frequency conversion circuit.
Consumer demand for smaller wireless products such as pagers and cellular phones continues to drive manufacturers to miniaturize circuit blocks in SCRs (selective call radios). FIG. 1 illustrates a classical structure of a prior art dual conversion radio receiver 100. This structure is commonly utilized in wireless communication products.
As shown, the dual conversion receiver 100 consists of an antenna 102 coupled in series with a low-noise amplifier 104 and a preselector filter 106. The preselector filter 106 is essentially a bandpass filter for attenuating undesired signals that are far from a desired signal in the frequency spectrum. This filtered signal is down-converted by a first mixer 108, which is coupled to an injection frequency signal from a first LO (local oscillator) 110. Once the filtered radio signal has been down-converted to an IF (intermediate frequency) signal, the IF signal is again filtered by a selectivity filter 112 to substantially attenuate undesired signals close in frequency to the desired signal. The filtered signal generated by the selectivity filter 112 is then down-converted by a second mixer 114 coupled to a second LO 116, which generates a baseband signal 118.
Presently, a large portion of the circuits of the first mixer 108 and the first local oscillator 110 are constructed with discrete electrical components, rather than integrated into one or more IC (integrated circuit) components. Similar to the first LO 110, the manufacture of the second mixer 114 and the second LO 116 also consists of discrete circuit components for generating the second injection frequency. Thus, neither the first nor the second stage of frequency conversion circuits have been fully integrated into an IC in prior art systems. Historically, designers of portable radio units have avoided the integration of the foregoing circuits, especially in narrow-band applications, due to the high side band noise resulting from the integration of local oscillators, which in turn limits the ACIPR (Adjacent Channel Interference Protection Ratio) of the receiver.
In view of a desire for miniaturizing wireless products in the consumer market, a need exists in selective call radios for integrating a frequency conversion circuit into an IC, thereby overcoming the disadvantages described in the prior art.